Intellectual property rights endure for long periods. Under current European laws, patents last 20 years, registered designs up to 25 years, a copyright can easily last over 100 years and a trade mark may remain registered indefinitely. The law cannot remain static for such long periods, however. Changes in technology, business models and social attitudes, among other things, mean that intellectual property legislation must be periodically revised if it is to remain fit for purpose.2 Although the speed with which legislatures react to the need for change varies greatly, most countries do introduce new or amended laws from time to time. In addition, the desire for increased harmonisation, both internationally and regionally, is another factor that leads to legislative change. What happens when new legislation comes into force? If the legislation is creating an entirely new right, such as the Community trade mark (now EU trade mark) or Community registered design, then it is possible for the legislature to start with a clean sheet.3 More usually, however, there will be existing rights to which the legislation may apply. In these circumstances the legislature is faced with a series of choices. Should the new legislation only apply fully prospectively, that is to say, to rights that come into existence after the legislation comes into force? Or partially prospectively, that is to say, both to existing rights and to new rights, but in the case of existing rights only in respect of future acts of infringement? Or retrospectively (which may extinguish existing causes of action if the right is narrowed or turn lawful acts retrospectively into unlawful ones if the right is broadened)? Should there be some kind of phasing-in period? Should there be protection for people who have started an activity and may now be turned into infringers, and if so what form should such protection take? How should existing transactions such as assignments and licences of existing rights be treated? It is the function of transitional provisions to address these questions.4 In practice, transitional provisions in intellectual property legislation are of considerable importance, but legislatures frequently give them insufficient attention and the subject has been little studied by scholars. This chapter represents a small attempt to redress that neglect, although space does not permit a full treatment of this important subject.